1. Field of the Invention
The present invention relates to a recording and reproducing method and a recording and reproducing apparatus using a probe, for writing and reading information by physical interaction between the probe having a tip at its distal end and a recording medium opposed thereto.
2. Related Background Art
There is recently developed a scanning tunneling microscope (hereinafter referred to as STM) enabling to directly observe an electronic structure of surface atoms of a conductor, whereby high-resolution measurement of a real, spatial image became possible, whether for a single crystal material or for an amorphous material. [G. Binnig et al. Phys. Rev. Lett, 49, 57 (1982)]
The STM utilizes the fact that a tunnel current flows when a metal probe (tip) is brought close to a conductive material, for example up to a distance of about 1 nm, while applying a voltage between them. This current is very sensitive to a change of the distance between them. Scanning the surface with the probe so as to keep the tunnel current constant, even a variety of information concerning a total electron cloud in a real space can be read. For the measurement the resolution is about 0.1 nm in in-plane directions.
Accordingly, applying the principle of STM, high-density recording and reproduction can be readily effected in the atomic order (sub-nanometer order). For example, a recording and reproducing apparatus, as disclosed in Japanese Laid-open Patent Application No. 61-80536, is so arranged that atom particles adhered to a surface of a medium are removed with an electron beam to write data and the written data is reproduced by STM.
There are suggestions concerning a method which employs as a recording layer a material having memory effect for switching characteristics of voltage-current, for example a thin-film layer of a material selected from .pi.-electron organic compounds and chalcogen compounds, to record and reproduce data with STM (Japanese Laid-open Patent Applications No. 63-161552 and No. 63-161553). This method permits large-capacity recording and reproduction, for example at the density of 10.sup.12 bit/cm.sup.2 in case of the recording bit size being 10 nm. Further, some suggestions are directed to a reduction of the size, proposing apparatus in which a plurality of probes with respective tips are formed on a semiconductor substrate and a recording medium opposed thereto is displaced to record data (Japanese Laid-open Patent Applications No. 62-281138 and No. 1-196751). For example, if the above-described material having the memory effect is combined with a multi-probe head in which 2500 probes are arranged in a matrix of 50.times.50 on a 1-cm.sup.2 -square silicon chip, recording or reproduction of digital data can be made at recording capacity of 400 Mbit per probe or at total recording capacity of 1 Tbit.
In such recording and reproducing apparatus, signals are normally recorded in the form of bits on the surface of the recording medium and the recorded bits are detected in the form of a level change of the tunnel current (in case of STM) upon reproduction. For example, in case of binary signals being recorded, bits are arranged on the recording medium to effect recording with presence or absence of bit and the information is read out detecting the tunnel current change between the probe tip and the recording medium (Japanese Laid-open Patent Application No. 63-96756). Another suggestion concerns analog signal recording of the tunnel current according to an information-writing size, using a medium which selects a charge or a magnetic domain in the molecular size depending upon recording data, as a recording medium (Japanese Laid-open Patent Application No. 2-210633).
The following problems are, however, recognized in the above cases where the data is recorded with presence or absence of bit or with difference of signal level and the recorded data is reproduced by detecting the presence or absence of signal or the signal intensity difference from recorded bits.
(1) As described previously, STM or AFM (atomic force microscope) has high resolution of angstrom order in the vertical direction. Thus, if it was used to construct a recording and reproducing apparatus of nanometer order, the high resolution in the vertical direction caused a large change of detected signals even with noise components such as defects or fine vertical undulations on the recording medium. Then flatness and uniformity of atomic level was required for the recording medium to be used in recording and reproduction. It has been difficult to produce a recording medium satisfying this requirement over a wide area. Therefore, it has been desired to achieve a recording and reproducing method which could permit recording and reproduction at sufficient S/N ratio even with some disturbance of flatness of the recording medium. Such suggestions have been rare.
(2) In the case where the analog signal recording was carried out by changing a modulation amount of recorded information bits in accordance with writing information size, the shape of recorded bits relying on the recording information must be formed with good reproducibility and at accuracy of angstrom order. It has been, however, actually difficult to effect the analog recording while changing the shape of recorded bits accurately in the angstrom order on an analog basis.
(3) In the recording and reproducing method utilizing the principle of STM, servo operation is normally employed to keep signals of the tunnel current constant between the tip and the medium. In case of the binary recording, for example, original signals were not used without modulation for recording, but modulation was necessary for the recording signals not to have a dc signal component. This needed a modulation circuit of complex structure and a demodulation circuit for demodulating modulated data.